The present invention relates to a controller for cylinder cut-off for a multi-cylinder internal combustion engine which includes a partially operable cylinder that can be cut-off by a cut-off cylinder mechanism provided therewith and a full operating cylinder that is not cut off.
In controller for cylinder cut-off for attempting to decrease the fuel consumption by cutting off a cylinder of a multi-cylinder internal combustion engine of this type when the load of the engine is low, there is proposed such a controller for cylinder cut-off which controls such that the cut-off of the cylinder is prohibited when a temperature of a partially operable cylinder side catalyst provided along an exhaust passageway connecting to a partially operable cylinder is estimated, the partially operable cylinder side catalyst temperature so estimated becoming lower than a predetermined temperature of the catalyst (for example, refer to a patent literature No. 1).
[Patent Literature No. 1]
JP-A-2001-227369
In the event that the temperature of a catalyst provided on a partially operable cylinder side becomes lower than the activation temperature thereof due to the cut-off of the partially operable cylinder, since exhaust gas is not purified effectively until the catalyst is heated to the activation temperature thereof after the cylinder cut-off is released and hence exhaust emissions get worse, in the aforesaid patent literature No. 1, when the estimated catalyst temperature lowers below the predetermined temperature, the cut-off of the cylinder is prohibited, whereby the cylinder is reactivated, so that the catalyst is maintained activated at all times.
On the other hand, the full operating cylinder which is arranged to operate at all times while the partially operable cylinder is being cut-off takes over the output that would otherwise be borne by the partially operable cylinder, the temperature of the full operating cylinder side catalyst is increased by the heat of exhaust gas.
The catalyst itself has a heat-resistant critical temperature, and since there is a risk that the catalyst is damaged in case the catalyst is heated to exceed this critical temperature, the cut-off of the partially operable cylinder is released before then so as to allow all the cylinders to operate, whereby the load of the full operating cylinder is reduced, thereby making it possible to reduce the temperature of the full operating cylinder side catalyst.
As is described in the patent literature No. 1, in the event that the cut-off of the cylinder is controlled to be prohibited when the partially operable cylinder side catalyst temperature becomes lower than the predetermined value by monitoring only the partially operable cylinder side catalyst, there may occur a case where the temperature of the full operating cylinder side catalyst exceeds the heat-resistant critical temperature before the temperature of the partially operable cylinder side catalyst becomes lower than the predetermined temperature, resulting in a risk that the full operating cylinder side catalyst is damaged.